1. Technical Field
This invention relates generally to a method of controlling the injection of fuel into Diesel engines to selectively provide a rich fuel mixture, and more particularly to such a method in which a rich fuel mixture is used to provide fuel rich combustion products for the periodic regeneration of lean NOX traps.
2. Background Art
Worldwide emissions regulations slated for introduction in the near future will require that practically all internal combustion engines be equipped with some form of exhaust treatment device. Perhaps of most concern are the proposed reductions in NOX emissions because they are the most difficult to mitigate from the exhaust stream of fuel-lean combustion, typical of Diesel engine combustion. One of the most promising technologies for NOX treatment is a NOX adsorber, also known as a “Lean NOX Trap” (LNT). However, these devices generally require that products of fuel-rich combustion be periodically passed through the LNT catalyst in order to regenerate the NOX trapping sites and convert the released NOX into N2 and CO2.
For example, U.S. Pat. No. 5,732,554 issued Mar. 31, 1998 to Shizuo Sasaki, et al. for an EXHAUST GAS PURIFICATION DEVICE FOR AN INTERNAL COMBUSTION ENGINE describes a rich combustion control method for reducing soot emissions by lowering the amount of excess air while simultaneously increasing the quantity of pre-mixed fuel. However, it has been discovered that the reduction of excess air flowing through the engine, such as by increasing the exhaust gas recirculation rate, has an adverse effect on smoke production and fuel economy at higher engine loads.
U.S. Pat. No. 5,937,639 granted Aug. 17, 1999 to Shizuo Sasaki, et al. for an INTERNATIONAL COMBUSTION ENGINE describes a method for lowering the combustion temperature to minimize smoke generation during rich, or near rich, combustion. However, it has been found that lowering combustion temperature cannot be used effectively over all of a Diesel engine's operating regime. Dr. Shizuo Sasaki, a co-inventor of the above referenced patents is also a co-inventor of the present invention.
The present invention is directed to overcoming the problems set forth above with respect to providing rich pulse control of combustion over the entire operating regime of a Diesel engine. It is desirable to have such a method that avoids the problem of smoke production when operating at reduced air flow rates. It is also desirable to have such a method that concurrently avoids localized high combustion temperatures when operating in relatively low load regions of a Diesel engine's operating regime. Furthermore, it is desirable to have a method for providing rich pulse control that does not produce wide torque fluctuations and eliminates oil dilution caused by post fuel injection when operating at relatively low loads.